U.S. patent application number 12/034217 was filed with the patent office on 2009-03-05 for organic light emitting device including photo responsive material and a method of fabricating the same.
This patent application is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. Invention is credited to Pil-soo AHN, Hee-kyung KIM.
Application Number | 20090058272 12/034217 |
Document ID | / |
Family ID | 40406362 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090058272 |
Kind Code |
A1 |
KIM; Hee-kyung ; et
al. |
March 5, 2009 |
ORGANIC LIGHT EMITTING DEVICE INCLUDING PHOTO RESPONSIVE MATERIAL
AND A METHOD OF FABRICATING THE SAME
Abstract
Provided is a method of fabricating an organic light emitting
device using a solution process. The method includes forming an
electrode on a lower substrate; depositing an organic active
material solution containing at least one photoreactive material on
the electrode to form an organic active material layer; and
radiating light onto the organic active material layer so that a
characteristic of the light varies according to the depth of the
organic active material layer in order to gradually vary a
molecular orientation structure in the organic active material
layer according to the depths, thereby resulting in a carrier
mobility gradient according to the depths of the organic active
material layer.
Inventors: |
KIM; Hee-kyung; (Anyang-si,
KR) ; AHN; Pil-soo; (Suwon-si, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
SAMSUNG ELECTRONICS CO.,
LTD.
Suwon-si
KR
|
Family ID: |
40406362 |
Appl. No.: |
12/034217 |
Filed: |
February 20, 2008 |
Current U.S.
Class: |
313/504 ;
427/508; 427/553 |
Current CPC
Class: |
H01L 51/5048 20130101;
H01L 51/5012 20130101; H01L 51/506 20130101; H05B 33/10 20130101;
H01L 2251/5346 20130101; H01L 51/0076 20130101; H01L 51/5076
20130101; H01L 51/0039 20130101; H01L 51/56 20130101 |
Class at
Publication: |
313/504 ;
427/553; 427/508 |
International
Class: |
H01J 1/63 20060101
H01J001/63; B05D 3/06 20060101 B05D003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2007 |
KR |
10-2007-0089956 |
Claims
1. A method of fabricating an organic light emitting device, the
method comprising: forming an elecctrode on a lower substrate;
depositing an organic active material solution containing at least
one photoreactive material on the electrode on the lower substrate
to form an organic active material layer; and radiating light onto
the organic active material layer so that the characteristics of
the light varies according to the depth of the organic active
material layer in order to gradually vary the molecular orientation
structure in the organic active material layer according to the
depths, thereby resulting in a carrier mobility gradient according
to the depths of the organic active material layer.
2. The method of claim 1, wherein the at least one photoreactive
material comprises one of a photopolymerizable material, a
photoisomerizable material, and a photodecomposable material.
3. The method of claim 1, wherein the organic active material layer
includes at least one selected from among an organic emitting
material, an electron transporting material, and a hole
transporting material.
4. The method of claim 1, further comprising adding a p-type or
n-type dopant that increases electrical conductivity to the organic
active material layer.
5. The method of claim 2, wherein the least one photoreactive
material includes a photopolymerizable material, and the
photopolymerizable material is mixed into the organic active
material solution in the form of monomer and then polymerized into
a molecular orientation structure which varies depending on the
intensities of light radiated onto the organic active material
layer.
6. The method of claim 1, wherein the organic active material
solution further comprises a photoinitiator.
7. The method of claim 1, wherein the at least one photoreactive
material differently responds to light having different
characteristics, and in the radiating of light onto the organic
active material layer, lights having different characteristics are
radiated onto both surfaces of the organic active material layer
from opposite directions.
8. The method of claim 1, wherein, in the radiating of light onto
the organic active material layer, a coherent light source is used
to form an interference pattern of light with an intensity gradient
which varies according to the depth of the opposite active material
layer.
9. The method of claim 8, wherein two coherent light sources are
arranged on upper and lower surfaces of the organic active material
layer to face each other, and the two coherent light sources
radiate phase-adjusted light to form the interference pattern
within the organic active material layer.
10. The method of claim 8, wherein one coherent light source is
arranged on an upper or lower surface of the organic active
material layer, whereas a reflective layer is formed on the other
surface of the organic active material layer on which the coherent
light source is not arranged, and the coherent light source
radiates phase-adjusted light to form the interference pattern
within the organic active material layer.
11. The method of claim 8, wherein the relationship between the
thickness of the organic active material layer and the wavelength
(.lamda.) of light satisfies the condition that the thickness of
the organic active material layer is an n multiple of .lamda./4,
where n is a natural number.
12. The method of claim 7, wherein the different characteristics of
the lights radiated from upper direction and lower direction are
selected from the one or the combination of their intensity,
wavelength, polarization, and incident angle, respectively.
13. The method of claim 1, wherein the at least one photoreactive
material includes a material whose molecular orientation varies
depending on the characteristic of radiated light.
14. The method of claim 1, wherein the molecular orientation
structure of the organic active layer is varied to obtain a carrier
mobility gradient with a hole mobility which gradually decreases
from an anode toward a cathode and an electron mobility which
gradually increases from the anode toward the cathode.
15. An organic light emitting device comprising: an anode; a
cathode; and at least one organic active layer arranged between the
anode and the cathode, wherein the organic active layer includes at
least one material selected from among an organic emitting
material, an electron transporting material, and a hole
transporting material, and at least one photoreactive material and
the organic active layer has a molecular orientation structure
which gradually varies, resulting in a carrier mobility gradient
according to the depth of the organic active layer.
16. The organic light emitting device of claim 15, wherein the
organic active layer has a molecular orientation structure having a
carrier mobility gradient with a hole mobility which gradually
decreases from an anode toward a cathode and an electron mobility
which gradually increases from the anode toward the cathode.
17. The organic light emitting device of claim 15, wherein the at
least one photoreactive comprises one of a photopolymerizable
material, a photoisomerizable material, and a photodecomposable
material.
18. The organic light emitting device of claim 15, wherein the
photoreactive material is a photopolymerizable material, and
molecules of the photoreactive material polymerize to a degree of
polymerization which gradually varies according to the depth of the
organic active layer.
19. The organic light emitting device of claim 15, wherein the
photoreactive material is a photoorientable material, and molecules
of the photoreactive material orientate in a direction which
gradually varies according to the depth of the organic active
layer.
20. The organic light emitting device of claim 15, wherein
molecules of the photoreactive material are arranged to have an
order parameter which gradually varies according to the depths of
the organic active layer.
21. The organic light emitting device of claim 15, wherein the
organic active layer further comprises a p-type or n-type dopant
that increases electrical conductivity.
22. The organic light emitting device of claim 15, wherein the at
least one material selected from among an organic emitting
material, an electron transporting material, and a hole
transporting material is a photoreactive material.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application claims the benefit of Korean Patent
Application No. 10-2007-0089956, tiled on Sep. 5, 2007, in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein in its entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an organic light emitting
device (OLED) and a method of fabricating the same and more
particularly, to an OLED having an organic active layer with a new
molecular orientation structure and a method of fabricating the
OLED in which the organic active layer is formed using a solution
process.
[0004] 2. Description of the Related Art
[0005] Organic light emitting devices (OLED), which include an
anode, a cathode, and an organic active layer including a
fluorescent or phosphorescent organic compound between the anode
and the cathode, are self-emissive devices which spontaneously emit
light as holes supplied from the anode and electrons supplied from
the cathode combine with each other. Generally, the organic active
layer includes an organic emission layer (EML), a hole transporting
layer (HTL) and a hole injecting layer (HIL) between the EML and
the anode, and an electron transporting layer (ETL) between the EML
and the cathode.
[0006] In general, an OLED fabricated by thermal vapor deposition
has a heterojunction structure including an HIL, an HTL, an organic
EML, and a hole barrier layer (HBL), which are formed of
heterogeneous materials, in order to obtain an efficient emission
structure. In addition, in order to realize a large size OLED
display, thickness uniformity of the layers should be ensured.
However, it is difficult to form a large-sized organic active layer
having a uniform thickness by the conventional vapor deposition
process. Thus, recently a method of coating an organic material by
a solution process, which is a wet process, has been suggested. In
the solution process, a solution of an organic material, having a
high solubility, dissolved in a solvent is coated by spin coating,
inkjet coating, or the like. However, fundamentally this solution
process is not suitable for forming a multi-layer structure because
a solvent used to form an upper layer may melt a lower layer.
[0007] Meanwhile, Franky So (U.S. Pat. No. 5,925,980), J. J. Brown
(Proceedings of SPIE Vol. 4800 (2003)), C. Wu (Appl. Phys. Lett.
Vol. 86103506 (2005)), Yang Yang (Appl. Phys. Lett. Vol. 832453
(2003)), and others reported results on efficienty improvement or
lifespan increase when a graded junction forming method is used, in
which one of the layers in an OLED has the same composition as an
adjacent layer, but with a composition gradient.
SUMMARY OF THE INVENTION
[0008] The present invention provides an organic light emitting
device (OLED) with a high-efficiency, long-lifespan organic active
layer, and a method of fabricating the OLED, in which the overall
fabrication process is simplified by using a solution process and
the molecular arrangement structure in an organic active layer
gradually varies according to depth, so that a mobility gradient of
the mobility of holes and electrons varies gradually.
[0009] According to an aspect of the present invention, there is
provided a method of fabricating an organic light emitting device,
the method including forming a lower substrate on a substrate;
depositing an organic active material solution containing at least
one photoreactive material on the lower substrate to form an
organic active material layer, and radiating light onto the organic
active material layer so that a characteristic of the light varies
according to the depth of the organic active material layer in
order to gradually vary a molecular orientation structure in the
organic active material layer according to the depths, thereby
resulting in a carrier mobility gradient according to the depths of
the organic active material layer.
[0010] The at least one photoreactive material may include one of a
photopolymerizable material, a photoisomerizable material, and a
photodecomposable material. The the organic active material layer
may include at least one selected from among an organic emitting
material, an electron transporting material, and a hole
transporting material. The method may further include adding a
p-type or n-type dopant that increases electrical conductivity to
the organic active material layer. The the organic active material
solution may further include a photoinitiator.
[0011] According to another embodiment of the present invention, a
method of fabricating an OELD may include forming an organic active
material layer as a thin film by depositing a mixed solution
containing a photoreactive material and radiating light onto the
organic active material layer, wherein the intensity of light
varies according to the depth of the organic active material layer
so that a molecular orientation structure (for example, the degree
of polymerization, molecular orientation, or order parameter) also
varies according to the depth of the resulting organic active
layer.
[0012] For example, the at least one photoreactive material may be
a photopolymerizable material. The photopolymerizable material may
be added in the form of a monomer into the solution for forming the
organic active material layer so that the photopolymerizable
material in the form of a monomer polymerizes into a molecular
orientation structure with a degree of polymerization that varies
depending on the intensities of light that has reached the organic
active material layer. The degree of polymerization varies with the
depth of the organic active material layer. The photopolymerizable
material may be a carrier transporting material or an
electroluminescent material.
[0013] In an exemplary method of gradually varying the intensity of
light reaching the organic active material layer, when light is
radiated onto the organic active material layer, a coherent light
source may be used to form an interference pattern of light with an
intensity gradient which varies according to the depth of the
opposite active material layer. In particular, two coherent light
sources may be arranged on upper and lower surfaces of the organic
active material layer to face each other, and the two coherent
light sources may radiate phase-adjusted light to form the
interference pattern within the organic active material layer.
Alternatively, one coherent light source may be arranged on an
upper or lower surface of the organic active material layer,
whereas a reflective layer is formed on the other surface of the
organic active material layer on which the coherent light source is
not arranged, and the coherent light source radiates phase-adjusted
light to form the interference pattern within the organic active
material layer.
[0014] In the case of varying the intensity of light according to
the depth of the organic active material layer using an
interference pattern as described above, the relationship between
the thickness of the organic active material layer and the
wavelength (.lamda.) of light may satisfy the condition that the
thickness of the organic active material layer is an n multiple of
.lamda./4, where n is a natural number. However, the present
invention is not limited to this relationship.
[0015] In a method of fabricating an OLED according to another
embodiment of the present invention, the at least one photoreactive
material may respond differently depending on the characteristics
of light. When light beams having different characteristics are
radiated onto the organic active material layer, the light beams
having different characteristics are respectively radiated onto the
opposing surfaces of the organic active material layer from
different directions. The characteristic of light may be at least
one of intensity, wavelength, polarization, and incident angle.
Alternatively, the at least one photoreactive material may include
at least two materials which respond to light having specific
characteristics. In this case, when light is radiated onto the
organic active material layer, light beams having different
characteristics are radiated onto the opposing surfaces of the
organic active material layer from different directions.
[0016] The at least one photoreactive material may include a
material whose molecular orientation varies depending on the
characteristic of radiated light. The at least one photoreactive
material may be a material whose molecules are oriented to have an
order parameter which varies depending on the characteristic of
radiated light.
[0017] The the molecular orientation structure of the organic
active layer may be varied to obtain a carrier mobility gradient
with a hole mobility which gradually decreases from an anode toward
a cathode and an electron mobility which gradually increases from
the anode toward the cathode.
[0018] According to another aspect of the present invention, there
is provided an OLED including an anode; a cathode; and at least one
organic active layer arranged between the anode and the cathode,
wherein the organic active layer includes at least one material
selected from among an organic emitting material, an electron
transporting material, and a hole transporting material, and at
least one photoreactive material and the organic active layer has a
molecular orientation structure which gradually varies, resulting
in a carrier mobility gradient according to the depth of the
organic active layer.
[0019] The the organic active layer may have a molecular
orientation structure having a carrier mobility gradient with a
hole mobility which gradually decreases from an anode toward a
cathode and an electron mobility which gradually increases from the
anode toward the cathode. The at least one photoreactive comprises
one of a photopolymerizable material a photoisomerizable material,
and a photodecomposable material.
[0020] In an OLED according to another embodiment of the present
invention, the photoreactive material may be a photopolymerizable
material, and molecules of the photoreactive material polymerize to
a degree of polymerization which gradually varies according to the
depth of the organic active layer.
[0021] In an OLED according to another embodiment of the present
invention, the photoreactive material may be a photoorientable
material, and molecules of the photoreactive material orientate in
a direction which gradually varies according to the depth of the
organic active layer. Alternatively, molecules of the photoreactive
material may be arranged to have an order parameter which gradually
vanes according to the depths of the organic active layer.
[0022] In the OLEDs according to the present invention described
above, the organic active layer may further include a p-type or
n-type dopant that increases electrical conductivity. In addition,
the at least one material selected from among an organic emitting
material, an electron transporting material, and a hole
transporting material may be a photoreactive material.
[0023] In general, an OLED includes an organic emission layer (EML)
containing an organic emitting material between an anode and a
cathode. Optionally, the OLED may include a hole transporting layer
(HTL) between the EML and the anode and an electron transporting
layer (ETL) between the EML and the cathode. The organic active
layer described in the present invention may be one of the EML, the
HTL and the ETL depending on its component, or may a layer
simultaneously performing the functions of at least two of the
listed layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
[0025] FIG. 1 is a view for illustrating a method of varying or
differentiating the intensity of light absorbed by an organic
active layer according to depth;
[0026] FIG. 2A illustrates a case where the optical density in the
organic active material layer varies according to depth due to the
interference of single-wavelength beams radiated in opposite
directions;
[0027] FIG. 2B illustrates a case where the optical intensity
variation due to the interference between an incident optical wave
and a reflected optical wave in the organic active material layer
varies according to depth;
[0028] FIG. 3 illustrates a case where light having different
wavelengths are radiated onto an organic active material layer from
opposite directions;
[0029] FIG. 4 illustrates a state of molecular orientation of a
photoreactive material by the combination of circularly polarized
light and linearly polarized light radiated from opposite
directions;
[0030] FIGS. 5A through 5C illustrate examples of different
combinations of polarized light radiated onto an organic active
material layer from two different directions in the photocuring
process;
[0031] FIG. 6 illustrates an example of radiating light having
different intensities onto the both surfaces of an organic active
layer;
[0032] FIG. 7 illustrates an example of radiating light onto the
both surfaces of an organic active layer at different incident
angles;
[0033] FIG. 8 illustrates an OLED A having a hole transporting
layer (HTL) with a uniform charge mobility and an OLED B having an
HTL with a charge mobility gradient;
[0034] FIG. 9 illustrates applied bias voltage, current density,
and luminance characteristics (a) for the OLED A in FIG. 8 and
applied bias voltage, current density, and luminance
characteristics (b) for the OLED B in FIG. 8;
[0035] FIG. 10 illustrates an OLED according to an embodiment of
the present invention; and
[0036] FIG. 11 illustrates an OLED according to another embodiment
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0037] Embodiments of the present invention will be described with
reference to the appended drawings.
[0038] FIG. 1 is a view for illustrating a method of varying or
differentiating the intensity of light absorbed by an organic
active layer according to depth. Initially, an electrode 20 is
formed on a substrate 10. Here, the substrate 10 may be one of a
glass substrate or a transparent plastic substrate, but the present
invention is not limited thereto. The substrate may also be a
planar structure in which different functional structures are
formed. For example, when manufacturing an active matrix type OLED
display panel, a substrate may include an organic thin film
transistor (OTFT) structure. The electrode 20 may be any conductive
material thin film which can function as an anode or a cathode
depending on the applied voltage. In the current embodiment, the
substrate 10 and the electrode 20, both of which are optically
transparent, and the electrode 20 functioning as an anode are
described as an example. An example of a material for the
transparent electrode 20 includes indium tin oxide (ITO).
[0039] An organic active material layer 31 is formed on the
electrode 20 using a solution process. In other words, a solution
of various organic active materials dissolved in a solvent is
prepared, and the prepared organic active material solution is
coated in the form of a thin film using a wet process, such as spin
coating, inkjet printing and the like. The process of coating the
organic active material solution may be one of spin coating,
gravure printing, roll-to-roll processing, syringe injection, dip
coating, spray coating, relief printing, lithography printing,
flexography printing, and screen printing.
[0040] Throughout the specification the thin film in which
molecules are not yet arranged in a desired orientation is referred
to as the organic active material layer 31, and the thin film after
the arrangement of molecules is implemented is referred to as an
organic active layer (not shown). The organic active material
solution may contain at least one of the materials, such as a hole
transporting material, an organic emitting material, and an
electron transporting material, which will be disposed between an
anode and a cathode. All or some of the materials listed above, for
example, only the hole transporting material and the organic
emitting material, may be contained in the organic active material
solution. In the current embodiment, the latter case is described.
Here, the function of the final organic active layer is determined
depending on the kinds of materials contained in the organic active
material solution. In other words, the organic active layer may be
formed as a layer having one of the functions of a hole
transporting layer (HTL), an organic emission layer (EML) or an
electron transporting layer (ETL), or may be formed as a layer
performing at least two of the functions.
[0041] The organic active material solution contains at least one
photoreactive material which may be photoorientable material. The
reaction of the photoreactive material to light may be
photopolymerization, photoisomerization, or photodecomposition. The
reaction of the photoreactive material varies depending on the kind
of the photoreactive material. Any photoreactive materials whose
molecular arrangement structure can be varied by the irradiation of
light, irrespective of the type of photoreaction involved, can be
used. Such photoreactive materials may be a charge transporting
material, an electron emitting material, or a material forming a
matrix of the organic active layer.
[0042] In the current embodiment a photoreactive material including
photopolymerization will be described in detail. Examples of such
materials including photopolymerization include
poly-TPBOX(N,N'-bis-[4(3-ethyl-oxetane-3-yimethoxy)-methylphenyl]-N,N'-bi-
s(phenyl)-benzidine]. This material is dissolved in an organic
solvent in the form of monomer and is photopolymerized by the
irradiation of ultraviolet (UV) light. However, the material is not
dissolved in an organic solvent in the form of polymer. This
poly-TPBOX in the form of polymer is known as a hole transporting
material. Examples of such materials which are photopolymerizable
and which can transport holes include materials having an oxetane
terminal group, for example, 3-ethyl-3-hydroxymethyloxetane,
1-(4-Bromophenyl)-6-bromohexane, 3-[6-(4-Bromophenyl)
hexyloxymethyl[-3-ethyloxetane,
N,N'-Di{4-[6-(3-ethyloxetane-3-yl-methoxy)]hexylphenyl}-N,N'-diphenylbenz-
idine.
[0043] According to an exemplary embodiment, such a hole
transporting, photopolymerizable material is dissolved in a solvent
together with the organic emitting material in order to prepare an
organic active material solution. Next, the organic active material
solution is coated by spin coating to form the organic active
material layer 31. As illustrated in FIG. 1, light, for example, UV
light, with an appropriate intensity, is then radiated into the
organic active material layer 31 through the substrate 10. In
addition in order to facilitate photopolymerization, a small amount
of a photoinitiator, for example, of no more than 0.1%, may be
added. Examples of such a photoinitiator include
{4-[(2-hydroxytetradecyl)-oxyl]-phenyl)-phenyl iodonium
hexafluoroantimonate and the like. In radiating light, the optical
intensity may be controlled so that the intensity of light reaching
inside the organic active material layer 31 has a gradient profile
in which the optical intensity decreases withgetting closer to the
surface of the organic active material layer 31, as illustrated in
FIG. 1. As a result, in the organic active material layer 31, the
polymerized hole transporting material has a gradient density
distribution. In other words, in a region of the organic active
material layer 31 which is close to the electrode 20 the
polymerization density of the photopolymerizable, hole transporting
material increases, resulting in a molecular arrangement suitable
for hole injection. However, in a region of the organic active
material layer 31 which is away from the electrode 20, the
polymerization density of the photopolymerizable, hole transporting
material decreases while the proportion of the organic emitting
material increases. As a result, the organic emitting material is
mainly distributed in the uppermost region of the organic active
material layer 31.
[0044] Through this process, in the cured organic active layer, a
gradient junction, in which the composition of the hole
transporting material and the organic emitting material
continuously varies, is formed. As a result, the organic active
layer has a gradient profile in carrier mobility, other electrical
characteristics, such as electrical resistance, and optical
characteristics. This means that the efficiency and lifespan of the
organic light emitting device (OLED) can be improved. The
relationship between such a gradient junction and the efficiency
and lifespan of organic light emitting devices was identified with
a low molecular weight OLED having a gradient junction
structure.
[0045] Although not illustrated in FIG. 1, after the organic active
layer with a gradient distribution of the hole transporting
material and the organic emitting material is formed as described
above, an additional organic active layer with a gradient
distribution of the organic emitting material and an electron
transporting material may be formed by a similar method as
described above on the organic active layer.
[0046] Meanwhile, in order to effectively induce an optical
intensity gradient in the organic active material layer 31, the
organic active material layer 31 may be formed to be thicker than
common organic active layers. However, the thickness of the organic
active layer increases, it may need to increase the driving
voltage. In order to prevent this, an n-type or p-type dopant may
be added to the organic active layer.
[0047] As an example, a method of inducing an optical intensity
gradient while maintaining the thickness of the organic active
layer at hundreds of nanometers will be described below. Unlike the
embodiment described with reference to FIG. 1, in which a common UV
light source is used, a coherent light source, for example, a laser
source, may be used in the embodiment described below.
[0048] FIG. 2A illustrates a case where the optical density in the
organic active material layer varies according to depth due to the
interference of single-wavelength beams radiated in opposite
directions. If is well known that optical coherence, and in
particular, constructive interference, can be induced by
controlling the phase of optical waves radiated from two coherent
light sources. As illustrated in FIG 2A, phase-adjusted beams are
radiated onto an organic active material layer 32 from opposite
directions in order to induce constructive interference between the
two beams in the organic active material layer 32. An interference
pattern as illustrated in FIG. 2A can be formed by controlling the
phrasal relationship between the two beams radiated from the
opposite directions and the distance between the two coherent light
sources and the organic active material layer 32. The optical
density gradient induced by such an interference pattern can lead
to a gradient molecular orientation in the organic active material
layer 32 as the organic active material layer 32 cures, as in the
embodiment described with reference to FIG. 1.
[0049] FIG. 28 illustrates a case where the optical intensity
variation due to the interference between an incident optical wave
and a reflected optical wave in the organic active material layer
varies according to depth. When a surface of the organic active
material layer 32 has a high reflectivity, for example, when a
metallic upper electrode (generally, a cathode) 40 is formed on a
surface of the organic active material layer 32 in order to
manufacture a bottom emission organic light emitting device, a
single-wavelength beam may be radiated onto a surface of the
organic active material layer 32 with a lower reflectivity (for
example, onto a surface close to a substrate (not shown) and an
electrode (not shown)? In this case, the interference between the
incident optical wave and the reflected optical wave can be induced
by adjusting the phase of the beam and the distance between the
light source and the organic active material layer 32. In other
words, an interference pattern can be induced in the organic active
material layer 32, as illustrated in FIG. 2B. This leads to a
gradient in the molecular orientation of the organic active
material layer 32 as the organic active material layer 32
cures.
[0050] As in the two examples described above, when a gradient
optical density is induced in the organic active material layer 32,
the relationship between the thickness of the optical active
material layer 32 and the wavelength (.lamda.) of radiated light
may satisfy the condition that the thickness of the optical active
material layer 32 is equal to approximately .lamda./4. However, the
thickness of the organic active material layer may be equal to an n
multiple of .lamda./4, where n is a natural number, depending on
the type of reaction of the photoreactive material in the organic
active material layer 32 to the optical intensity.
[0051] FIG. 3 illustrates a case where light having different
wavelengths are radiated onto an organic active material layer from
opposite directions. For example, when an organic active material
layer 33 contains at least two photoreactive materials which are
reactive to different wavelengths of light, light having the
different wavelengths, which are reactive with the photoreactive
materials, may be radiated onto the organic active material layer
33 from the opposite directions. In particular, the organic active
material layer 33 may contain a mixture of a hole transporting,
photopolymerizable material that is polymerizable by light having a
first wavelength (hereinafter, "optical wave 1"), an electron
transporting, photopolymerizable material that is polymerizable by
light having a second wavelength (hereinafter, "optical wave 2"),
and an organic emitting material. Here, the optical wave 2 is
radiated from a direction close to the electrode 20, which is an
anode, whereas the second wave 1 is radiated from the opposite
direction. As a result, the hole transporting material is
polymerized at a high density in a region of the resulting organic
active layer close to the anode, whereas the electron transporting
material is polymerized at a high density in a region of the
organic active layer close to the cathode. In addition, in the
middle of the organic active layer the organic emitting material is
distributed at a high density.
[0052] In the current embodiment, the method of radiating light
having different wavelengths onto the opposing surfaces of the
organic active material layer 33 from different directions has been
described as an example. The present invention is not limited to
this example, and similar results as this example can be obtained
by radiating light having different intensities, different
polarization characteristics or different incident angles onto the
opposing surfaces of the organic active material layer depending on
the used photoreactive material.
[0053] In another embodiment of the present invention, using a
photoreactive material whose molecular orientation direction is
determined depending on a characteristic (for example, wavelength,
intensity, polarization characteristic, or incident angle) of
radiated light, the molecular orientation direction in the organic
active layer of an OLED can be gradually varied according to the
depth of the organic active layer. For example, in a region of the
organic active layer, charge transporting molecules are arranged in
a direction parallel to the direction in which an electric field is
applied to the OLED, thereby increasing the mobility of holes. In
another region of the organic active layer, the charge transporting
molecules are arranged in a direction perpendicular to the
direction in which the electric field is applied, thereby lowering
the mobility of holes and raising the mobility of electrons.
[0054] FIG. 4 illustrates a state of molecular orientation of a
photoreactive material by the combination of circularly polarized
light and linearly polarized light radiated from opposite
directions. In the current embodiment, initially an organic active
material layer 34 is formed. The organic active material layer 34
includes a photoreactive material (in the form of monomer) whose
molecules are oriented depending on the polarization characteristic
of light radiated onto the electrode 20 (i.e., anode). During a
process of curing the organic active material layer 34, light
having different polarization characteristics is radiated onto the
organic active material layer 34 from the opposite directions so
that the photoreactive material is polymerized while being oriented
in an intended direction. Such a photoreactive material may be a
material which is mixed into the organic active material layer 34
in the form of monomer and which has liquid crystalline properties
at room temperature.
[0055] In order to photocure the organic active material layer 34
formed by spin coating, as illustrated in FIG. 4, circularly
polarized light may be radiated onto the organic active material
layer 34 from upward to downward, whereas linearly polarized light,
which is parallel to the substrate 10, may be radiated onto the
organic active material layer 34 from downward to upward. As a
result, in a region of the organic active material layer 34
adjacent to the substrate (anode) 20, molecules 301 of the
photoreactive material polymerize while being oriented in the
direction of the linearly polarized light, i.e., in the direction
parallel to the substrate 10. At this time, charge transporting
molecules doped into the organic active material layer 34 are
oriented in the same direction as the molecules of the
photoreactive material. Thus, by properly determining the direction
of linearly polarized light, the doped charge transporting
molecules can be oriented in a direction in which the hole
transporting ability is improved, to be suitable for hole injection
and transport. In a region of the organic active material layer 34
adjacent to an upper electrode (cathode, not shown), the molecules
301 of the photoreactive material polymerize while being oriented
in a direction perpendicular to the substrate 10 by the circularly
polarized light. At this time, the charge transporting materials
doped into the organic active material layer 34 are oriented in a
direction perpendicular to the substrate 10 to be suitable for
electron injection and transport. As a result, the organic active
material layer 34 is cured, resulting in a HTL region adjacent to
the anode that has a molecular orientation structure suitable for
hole transport, an ETL region adjacent to the cathode that has a
molecular orientation structure suitable for electron transport,
and an organic EML region between the HTL region and the ETL
region. In other words, an effective mobility gradient is formed in
the cured organic active layer.
[0056] As in the embodiment described above, the photoreactive
material may be a material forming a matrix of the organic active
layer. However, the present invention is not limited to this
example, and the photoreactive material may be a material having
electron emitting characteristics. For example, PFO
(poly[(9,9-dioctylfluoren-2,7-diyl]), which is an organic emitting
material, has a liquid crystalline property at room temperature
when it is in the form of a monomer. Of course, PFO can be used as
the photoreactive material.
[0057] FIGS. 5A through 5C illustrate examples of different
combinations of polarized light radiated onto an organic active
material layer from two different directions in the photocuring
process. Depending on the reaction characteristics of the
photoreactive material to polarized light, other combinations of
polarized light than the example in FIG. 4 can be applied. In other
words, for organic active material layers 35, 36, and 37, which
contain different photoreactive materials (having different
reaction characteristics to polarized light), various combinations
of polarized light which are suitable for the characteristics of
the photoreactive materials, can be radiated onto the opposing
surfaces of the organic active material layers 35, 36, and 37. FIG.
5A illustrates a combination of left-circularly polarized light and
linearly polarized light, FIG. 5B illustrates a combination of
linearly polarized light and right-circularly polarized light, and
FIG. 5C illustrates a combination of two linearly polarized light
beams perpendicular to each other.
[0058] Although the previous embodiments have been described with
reference to the photoreactive material which is reactive to the
polarization characteristics of radiated light, similar results as
above can be obtained by the combination of different wavelengths,
different intensities, or different incident angles of light
radiated onto the opposing surfaces of the organic active material
layer depending on the kind of the used photoreactive material.
[0059] FIG. 6 illustrates an example of radiating light having
different intensities onto the both surfaces of an organic active
layer. The molecular orientation structure of the photoreactive
material, for example, the degree of polymerization, density,
orientation, order parameter, and the like, of the photoreactive
material molecules varies depending on the intensity of light.
Thus, the mobility of charges can be changed by varying the
intensity of radiated light. A gentle gradient in the charge
mobility of the organic active material layer can be achieved by
varying the intensities of incident light radiated onto the
opposing surfaces of the organic active material layer depending on
the characteristics of the photoreactive material.
[0060] FIG. 7 illustrates an example of radiating light onto the
both surfaces of an organic active layer at different incident
angles. The molecular orientation structure in surface regions of
the organic active material layer varies depending on the incident
angle of light, thereby affecting the distribution of charge
mobility. Thus, a gentle gradient in the charge mobility of the
organic active material layer can be achieved by varying the
incident angles of light radiated onto the opposing surfaces of the
organic active material layer depending on the characteristics of
the photoreactive material.
[0061] FIG. 8 illustrates an OLED A having a hole transporting
layer (HTL) with a uniform charge mobility and an OLED B having an
HTL with a charge mobility gradient. FIG. 9 illustrates applied
bias voltage, current density, and luminance characteristics (a)
for the OLED A in FIG. 8 and applied bias voltage, current density,
and luminance characteristics (b) for the OLED B in FIG. 8. In the
OLED A in (a) of FIG. 8, there is no gradient in hole mobility, and
the OLED A has a stacked structure including, from bottom to top,
an anode 20, an organic active layer 38, which includes a HTL A
(having a thickness of 60 nm) and an ETL (having a thickness of 60
nm), and a cathode 40. In the OLED B in (b) of FIG. 8, there is a
gradient in hole mobility, and the OLED B has a stacked structure
including, from downward, the anode 20, an organic active layer 39,
which includes an HTL A (having a thickness of 30 nm), an HTL B
(having a thickness of 30 nm), and an ETL (having a thickness of 60
nm), and the cathode 40. In the OLED B, the HTL A has a hole
mobility of 1.times.10.sup.-6 cm.sup.2/Vs, and the HTL B has a hole
mobility of 1.times.10.sup.-4 cm.sup.2/Vs, indicating that the hole
mobility of the OLED B is higher closer to the anode 20. In
addition, as illustrated in FIG. 9, the OLED B shows a higher
current efficiency than OLED A, with a lower current density and a
higher luminance at the same applied voltage than the OLED A. This
comparative experiment shows that an OLED with a charge mobility
gradient has higher performance characteristics.
[0062] Hereinafter, exemplary embodiments of an OLED according to
the present invention will be described in detail.
[0063] FIG. 10 illustrates an OLED according to an embodiment of
the present invention. In the current embodiment, an organic active
layer 30 having both the functions of an HTL and an organic EML is
arranged between an anode 20 arranged on a substrate 10 and a
cathode 40. The organic active layer 30 includes a hole
transporting material and an organic emitting material, wherein the
hole transporting material may be a photopolymerizable material.
This organic active layer 30 may have, through the light radiating
process described with reference to FIGS. 1, 2A and 2B, a
distribution with a higher degree of polymerization of the hole
transporting material (designated by longer dashed lines) closer to
the anode 20 and a lower degree of polymerization of the hole
transporting material (designated by shorter dashed lines) closer
to the cathode 40. As a result, the HTL region of the organic
active layer 30 may have a gradient hole mobility varying according
to the depth of the organic active layer 30. Although the current
embodiment is described with reference to an example of using a
photopolymerizable material as the hole transporting material,
similar results as in the current embodiment can be obtained using
a material whose molecular order parameter varies by light
irradiation.
[0064] FIG. 11 illustrates an OLED according to another embodiment
of the present invention. The OLED according to the current
embodiment can be manufactured using the method described with
reference to FIG. 4. As illustrated in FIG. 4, the molecules 301 of
the photoreactive material are oriented differently in depths of
the organic active layer 34 depending on the characteristic of
light which has reached the depth. As a result, the hole
transporting material, the organic emitting material, and the
electron transporting material, which are included in the organic
active layer 34, may be distributed in the orientation direction of
the molecules 301 of the photoreactive material. Finally, a
molecular orientation structure suitable for hole injection and
transport may be formed in an HTL region close to the anode 20,
whereas a molecular orientation structure suitable for electron
injection and transport may be formed in an ETL region close to the
cathode 40. Since the molecules 301 of the photoreactive material
are oriented differently according to the depths of the organic
active layer 34, the mobility of charges also has a gradient
according to the depths of the organic active layer.
[0065] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
* * * * *